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Vocabulary Orogenesis –Oros—mountain –Genesis—to come into being Lithosphere –The rigid outer layer of Earth, including the crust and upper mantle Accretion –the increase in size of a tectonic plate by addition of material along a convergent boundary

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Crustal Uplift Evidence –Marine fossils often found in high elevations in mountains –Terraces hundreds of meters above sea level Isostasy –A floating crust in gravitational balance –Example: blocks of wood floating in water –Mountain belts stand higher above the surface of the Earth and have roots that extend deeper into the supporting material below. –Crustal thicknesses for some mountain chains are twice as much as the average for the continental crust

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Rock Deformation Elastic deformation –When stress is applied, rocks bend, but will snap back if the stress is relieved Plastic deformation –When the elastic limit is surpassed, rocks deform plastically or break (earthquakes) –They are permanently altered through folding and flowing

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Rock Deformation Folds –When flat-lying sedimentary and volcanic rocks are bent into a series of wavelike undulations Example: pushing on one edge of a carpet until it folds –Anticline Upfolding or arching of rock layers –Syncline Downfolds, or troughs

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Rock Deformation Faults and Joints –Fractures in the Earth’s crust –Dip-slip faults Vertical movement Hanging wall—rock that is higher than the fault surface Footwall—rock that is lower than the fault surface Normal—hanging wall moves downward relative to the footwall Reverse—hanging wall moves upward relative to the footwall –Thrust faults—have a very low angle

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Rock Deformation –Strike-slip faults The dominant displacement is along the strike or trend, of the fault (horizontal) Transform faults—associated with plate boundaries –Oblique-slip faults—both vertical and horizontal movement –Tensional forces—pull the crust apart Graben—central block bounded by normal faults; drop as the plates separate Horsts—upfaulted structures that are adjacent to graben –Compressional forces—sections of crust are displaced toward one another

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Rock Deformation –Joints Fractures along which no appreciable displacement has occurred –Columnar joints form when igneous rocks cool and develop shrinkage fractures, producing elongated, pillarlike columns –Sheeting produces a pattern of gently curved joints that develop more or less parallel to the surface of large exposed igneous bodies.

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Mountain Building –Andean type subduction zones Passive continental margin—part of the same plate as the adjoining oceanic crust Becomes active—subduction zone forms and the deformation process begins The oceanic plate descends and becomes magma while there is an accumulation of sedimentary and metamorphic rocks along the subduction zone (accretionary wedge)

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Mountain Building Mountain Building and Continental Accretion –Smaller crustal fragments collide and accrete to continental margins –Example: mountainous regions rimming the Pacific As oceanic plates move, they carry with them embedded oceanic plateaus or microcontinents The upper portions of these thickened zones are peeled from the descending plate and thrust in relatively thin sheets onto the adjacent continental block. This increases the width of the continent Terrane—accreted crustal blocks